Pulse time modulation system



April l5, 1949. H. GOLDBERG v PULSE TIME MODULATION SYSTEM 2 sneet-sheet1 Filed Feb. 9, 1946 IN VEN TOR.

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April 5, 1949. H. GOLDBERG PULSE TIME MODULATION S YSTEM 2 Sheets-Sheet2v Fled Feb. 9, 1946 no my. y Tw m@ M mm n 0 H m H RVA.

Patented pr. 5', 1949 UNITED STATES ATENT OFFICE PULSE TIME MODULATIONSYSTEM Harold Goldberg, Baltimore, Md., assgnor to Stromberg-CarlsonCompany, Rochester, N. Y., a corporation of New York ApplicationFebruary 9, 1946, Serial No. 646,615

7 Claims. i

This invention relates to a method of and to apparatus for push-pull,pulse period transmission and reception of signals.

In prior pulse modulation systems, the average value of the generatedpulses is not constant but is a function of the modulation of the signalto be transmitted. This condition does not afford the degree of secrecysometimes required, since an ordinary receiver can detect thetransmitted modulation and decipher it, although the resulting signalwill be somewhat distorted.

The main feature of the invention relates to a method of and toapparatus for Comunication in which the signal is transmitted in theform of a train of pulses coded according to an exponential function ftime as a result of sampling the modulating signal, the polarity of themodulating signal being reversed after each sampling, the instantaneousamplitudes being equal. Thus, there is provided pulse modulation inwhich the sum of every two pulse periods is a constant, eX- cept forsecond order effects, so that unauthorized deciphering of the' codedmessage is rendered more diflicult.

The invention will best be understood from the following descriptionwhen taken with the drawings in which:

Fig. 1 is a chart useful in explaining the coding and decodingprinciple;

Fig. 2 is a diagrammatic showing of the coding and transmitting portionof the present signalling system;

Fig. 3 is similarly a diagrammatic showing of the reception and decodingportion of this system; and

Fig. 4 is a chart showing certain wave forms present in the decodingportion of the system.

The principle of the coding and decoding system of the present inventioncan best be understood by reference to the chart of Fig. 1, whichillustrates one embodiment of my invention. In this chart curve -|-(t)-|H/2 represents the modulation voltage representing the information tobe transmitted by the coder and nally reproduced by the decoder at thereceiving station and the curve -f(t) +H/2 represents a voltage equal tof(t)|H/2 in amplitude at any given instant but opposite in sense.According to this invention samples only of the signal are transmitted,the sampling being effected at intervals determined by the intersectionof an exponentially decaying curve EG) and the modulating signal curvef(t)-|H/2. The sense of the modulating signal is reversed after eachsampling, as hereinafter described. The effect of this reversal isindicated in Fig. 1 by means of curves -l-kf(t) -l-H/Z and -lcf(t) |H/2,these curves representing the envelopes of the corresponding voltages.It will be understood, however, that actually pulses of alternatelyopposite sense will be derived from the cathode followers 9 and l0 (Fig.2). If -l-kf(t) is sampled at time tn then -lcf(t) is sampled at timetft-i-l and |lcf(t) is sampled at time tn-l-Z while kf (t) is sampled attime tn-l-S, etc. In this system, the sum of two adjacent periods in aconstant except for second order effects, an advantage if greatersecrecy is desired. It is preferred that the signal have amplitudesbetween the limits O and H in the coder to be described. For music orspeech this is not objectionable since multiplying by a constant or theadding of a constant does not constitute a distortion. The curve E05)has a specific time constant which is not restricted by the coding ordecoding arrangement. However, in a pulse modulation system which isbased on this coding and decoding principle, the time constant must besuch that, in the absence of the signal when a line of height l-l/2 issampled, the sampling rate must be at least twice the highest frequencycomponent in any signal which is to be sampled by the device.

The function +f(t)+H/2 is sampled in the following manner. Theexponential Eit) starting from height H decays until it intersectsfunction -|-f(t){-H/2. Such an intersection is indicated at time tn. Attime tn a pulse is generated and EU?) is instantaneously restored toyheight H. E(t) now decays exponentially until it intersects -f(t)+H/2,at tft-1 1. At this instant, another pulse is generated and E(t) isagain restored to the value H. This method is continued during thesignalling period. rhe pulses generated at each intersection nowrepresent the result of coding signal if(t){-H/2 and may be transmittedby any means known to the art. It may be seen that the interval betweenany two consecutive pulses is a measure of the amplitude of ii t +H/2 atthe end of the interval and this measure is according to the codingfunction EU).

Decoding is represented by the same chart. An exponential function Edt)is started from a height H, and allowed to decay. It is started by apulse, such as at time tn, which has been transmitted and received bymethods already known in the art. It is allowed to decay until the pulseat tn-|1 is received at which time it is again restored to height H,etc. The intervals between successive received pulses are the same asthose between the transmitted pulses but the individual pulses suffer afixed time delay in transmission. If the time constant of Elfi) thedecoder is the same as that in the coder, then the minima of theresulting wave produced hy EG) and its restoration to height El at eachpuise all lie on curves which are replica ci' it) -l-H/2 except for amultiplicative or addtive constant. The detection of the envelope of theoutput of the decoder i'()-;-H/2 may Toe done by means already wellknown in the In practicing the present method, the transmittingapparatus of Fig. 2 may be employed. The transmitting portion of thesystem comprises a microphone 5 to pick up the signal to be transmitted.This signal is amplified in the audio amplifier 6 and is suppliedthrough the transformer I to a coder unit which samples the function oftime to be transmitted, in the manner described above.

Generally this coder comprises two cathode followers including thetriodes il and lll which function together as an electronic switch. Thecoder also includes a blocking oscillator which comprises triode Il andwhich is controlled by the output of the cathode followers. The coderalso includes a scale-of-two counter comprising the triodes i3 and Hl,which counter is governed by the output of the oscillator to generate arectangular wave I5, the polarity of which changes which operates thescale-of-two counter is de- 'fv layed slightly in time, from the pulsewhich is transmitted as the output of the coder. This delay isaccomplished since the output pulses of the blocking oscillator are notunidirectional but have a considerable overshoot as indicated at p. Theovershoot is used to trigger the scaleof-two counter. This countercircuit, in addition to controlling the switching of the followers, alsofunctions to supply the biasing voltage for the followers. The followersare switched due to the fact that any great inequality in the averageVoltage applied to the two grids will cut off one of them.

Specincally, the coder may be described as follows: the triode 9 of oneof the cathode followers is provided with the cathode il, control gridI8 and anode iS while the triode Il) of the other cathode follower isprovided with the cathode 20, control grid 2l and anode 22. The cathodesI1 and 20 are connected together and to ground through resistor 23, thepurpose of which will be described. As long as the cathode followertriodes 9 and lil are operating in their class A region, resistor 23 hasno effect and does not hinder the operation of the followers to pass thesignal ilcfhf). The control grid i3 is connected in series with theresistor 24, secondary winding 25 of the transformer l, and conductor 26to terminal 21 of the scale-of-two counter. The control grid 2l issimilarly connected in series with the resistor 28, secondary winding 29of the transformer l, and conductor 3D to the terminal 2l'a of thescale-of-two counter. Winding 29 is connected in reverse phaserelationship with respect to winding 25.

The triode l l, the transformer 3l, the RC combination of resistor 32and condenser 33, the cathode followers 9 and lll and the resistor 23constitute a blocking oscillator circuit, which produces a recurringvoltage for the initiation of pulses.

In the absence of a modulation i(t)-l-H/2, the blocking oscillatoroperates at a constant rate determined by the characteristics of thetriode Il, transformer 3i, the RC constant of resistor 32 and condenser33, and the voltage across resistor 23 as well as the biasing voltagesapplied to the control grids i8 and 2l of the cathode followers. Theaction of the blocking oscillator is such that it goes into violentoscillation if the voltage on grid 32 of triode il exceeds a certainvalue which is termed the firing Voltage. When the firing voltage isexceeded, the condenser 33 receives a large negative charge. Theoscillation of the blocking oscillator collapses and the negativevoltage on condenser 33 prevents further oscillation of the oscillator.The negative charge on the condenser 33, however, will leak off at arate determined by the product RC. As soon as the grid voltage of theoscillator has exceeded the firing voltage, this operation is repeated.At each oscillation, a pulse is generated. The interval between rings isa function of the output of the followers.

The pulse generated may either be radiated or transmitted overconductors in the well-known manner. However, as herein indicated, eachpulse is applied to the grid 36 of a triode 31 connected as a cathodefollower. The output of the cathode follower is applied through thecondenser 38 to the grid 39 of a gas-filled discharge tube 40 of thethyratron type, included in a triggered power supply circuit. Aninductance coil il is connected between the anode l2 of tube 40 and asource of high voltage. An artificial line comprising inductors 45 andcondensers 46 connected in series parallel is arranged to be chargedfrom the high-voltage source. When the tube 40 is rendered conducting bythe application of a pulse to its grid 39, the artificial linedischarges through the space between the anode 42 and the cathode i3 ofthe discharge tube, inductance coil 4l preventing the high-voltagesource from being short-circuited. The output of the artifcial line isdelivered through a transformer 4l, to a well-known oscillatortransmitting unit 48 and the antenna 49.

If the signal-modulated pulses are radiated, as indicated, they areintercepted on an antenna 59, at a receiving station (Fig. 3) where theyare recreated by a suitable receiverinto signals corresponding to theoriginal signal. Specifically, the receiver is herein illustrated ascomprising a radio frequency stage 5l in which the received pulses areamplified and are then delivered to a local oscillator and mixer 52 totranslate the signal into an intermediate frequency. These intermediatefrequency signals are amplified by the intermediate frequency amplifier5d' and thereafter are detected preferably by a well known diodedetector 55. The detected signals are then passed through a vacuum tubelimiter or clipping stage 55 so that all signals derived from thereceiver are positive pulses of constant amplitude.

These positive pulses are supplied through the winding 5S of the`transformer 53 to an envelope detector which serves as a push-pullpulse period demcdulator and functions todetect the envelopes ofalternate minima in the output of the decoder and to combine theresulting push-pull output. The envelope detector includes a triggeredblocking oscillator comprising a triode 60 with its cathode 6l, its grid62 and anode 63, to act as a pulse amplifier, sharpener, and clipper.The network including the resistor B4 in the cathode lead provides abias voltage for the grid S2 which normally keeps the oscillator circuitfrom oscillating. A positive pulse from the receiver triggers theoscillator circuit and causes it to generate a single blockingoscillator pulse. These oscillator pulses are of constant amplitude andbecause of their form it is possible also to obtain a delayed pulse byreversing the sense of the output pulse from the transformer. Thus,there is provided a method for obtaining a delayed pulse without the useof delay networks. The delayed pulse is applied over conductor 6l totrigger a decoder, including triode 58 as well as to control ascale-oftwo counter comprising the triodes 69 and lil. The abovedescribed envelope detector is described and claimed in a co-pendingapplication of myself and James A. Krumhansl, Serial No. 646,616, iiledFebruary 9, 1946, and assigned to the same assignee as the presentinvention.

The triode 68 has its anode 'l2 connected to a source of potentialpositive with respect to ground and has its grid 13 connected to thedelayed pulse conductor 61. The cathode Ml of the triode has a resistor'l5 shunted by a condenser 'i6 connected between it and ground, andthese elements constitute an RC network which has a product equal tothat of the resistor 32 and the condenser 33 at the transmitter. It willbe understood that a positive pulse applied to the grid I3 of thedecoder causes the condenser I6 to charge to a fixed Value. However, thesubsidence of the positive pulse results in the cut-off of triode 68 sothat condenser 16 discharges exponentially through resistor 15 asindicated graphically in Fig. 1 by the curve EG).

The scale-of-two counter, comprising the triodes 69 and 16, functionsafter the manner of an Eccles-Jordan circuit to produce two rectangularwave outputs. These outputs are capacitively coupled, by the condensers16 and 19, to the diodes 86 and 8i respectively.

These diodes act as direct current restorers and give rise to the Waveforms B and C (Fig. 4) on the diode plates 32 and 83 respectively whilethe graph A illustrates the correlated pulse input to the counter. Itwill be seen that the voltages on the diode plates 82 and 63 alternatewith each other and are never positive. To each of these voltages, thereis added a normal pulse from transformer windings 85 and 86 respectivelyand the combined voltages are respectively applied to the grids 6l and88 of the envelope detector 89, 90 as Well as to the grids 9| and 92 ofthe envelope detectors 93 and 94. In this manner, the two envelopedetectors are activated alternately and each by itself determines thealternate minima of the decoder. The outputs of the two detectors aretherefore push-pull versions of the original modulation. The twodetectors are respectively coupled by the condensers 96 and 91 tocathode followers including the triodes 98 and 99. The transformerwinding lill) in the cathode circuits of the followers 98 and 99 combinethe push-pull outputs, the resultant being applied through a secondarywinding lill of the transformer to a low-pass filter |62 and thenamplified in the audio amplifier |03. The decoded signals, thusamplified, are reproduced by the loud speaker LS.

What I claim is:

1. In a pulse modulation system, a source of periodically recurringvoltage, a source of modulation voltage, means for producing anadditional voltage equal in amplitude to said modulation voltage but ofopposite sense, and means for alternately causing said modulationvoltage and said additional voltage to control the periodicity of saidrecurring voltage according jointly to an exponential function of timeand to the corresponding amplitudes and sense of said modulation andadditional voltages.

2. In a pulse modulation system, a source of signal voltage, means forproducing an additional voltage equal in amplitude to said signalvoltage but of opposite sense, means for alternately sampling saidvoltages at intervals varying as an exponential function of time and asa function of the respective amplitudes of said voltages, means forgenerating pulses at intervals corresponding to said sampling intervals,and means for transmitting said pulses as a train of pulses variablypositioned as to time.

3. In a pulse modulation system, a blocking oscillator, a source ofvoltage corresponding to information to be transmitted, means forproducing an additional voltage equal in amplitude to the firstmentioned voltage but of opposite sense, means for alternately utilizingsaid voltages for causing operation of said blocking oscillator atvariable intervals said intervals also depending on an exponential timefunction, and means responsive to each operation for transmitting apulse, the resulting pulses being positioned as to time according bothto said exponential time function and to the amplitudes an-d sense ofsaid voltages.

4. In a pulse modulation system, a blocking oscillator, a source ofvoltage corresponding to information to be transmitted, means forproducing an additional voltage equal in amplitude to the firstmentioned Voltage at any given instant but of opposite sense, a pair ofelectron discharge devices connected as cathode followers and eachhaving an anode, a cathode and a control electrode, said cathodes andanodes being connected for parallel operation, means for impressing thefirst mentioned voltage on the control electrode of one of said devices,means for impressing the second mentioned voltage on the controlelectrode of the other of said devices, means for alternately renderingsaid devices inoperative whereby a single output of pulsating voltage isobtained, means utilizing said output in combination with an exponentialtime function for controlling operation of said blocking oscillator, andmeans re- -sponsive to each operation for generating and transmitting apulse.

5. The method of communication which comprises originating a firstvoltage corresponding to a signal to be communicated, producing a secondvoltage at all times equal in amplitude to said rst voltage but ofopposite sense, alternately sampling said voltages at intervals varyingas an exponential function of time and as a function of the amplitude ofthe voltage being sampled, generating pulses at intervals correspondingto said sampling intervals, and transmitting the generated pulses as atrain of pulses variably posi' tioned as to time.

6. In a pulse modulation system, a -source of signal voltage, means forproducing a second voltage which at any given instant is equal inamplitude to said signal voltage but of opposite sense, means providinga D. C. voltage, means for combining one of the aforesaid voltages andsaid D. C. voltage to obtain a rst composite voltage, means forcombining the othei1 of the aforesaid `voltagesfand said DjC. voltage toobtain a second composite voltage, means for alternately samplingsaid-:composite voltages at intervals varying as an exponential functionof time and as a function of the respective amplitudes of said compositevoltages, means for generating pulses at intervals corresponding to saidsampling intervals, and means for transmitting said pulses as a train ofpulses Variably positioned as to time.

7. In a pulse modulation system, a source of signal voltage, means forproducing a second voltage which at any7 given instant is equal inamplitude to said signal voltage :but of opposite sense, means providinga D. C. voltage, means for Vadding one of the aforesaid voltages to saidD. C.

voltage to obtain a first composite voltage, means for subtracting theother of the aforesaid voltages from said D. C. Voltage to obtain asecond comiposite voltage, means for alternately sampling said compositevoltages at intervals varying as an eX- S ponential 'function of 'timeand las -a function'f the respective amplitudes of saidvcompositeVoltagesjmeans for generating pulses at intervals corresponding to saidsampling intervals, and means for transmitting said pulses as at'rain ofpulses Variably positoned as to time.

HAROLD GOLDBERG.

REFERENCES CITED The following references are of record in the ie ofthis patent:

UNITED STATES PATENTS Number Name Date 1,672,215 Heising June 5, 19282,068,918 Luck July 13, 1937 2,289,564 Wrathall July 14, 1942 2,401,384Young June 4, 1946 2,404,306 Luck July 16, 1946 2,412,964 Chatterjea etal. Dec. 24, 1946 2,416,329 Labin et al. Feb. 25, 1947 2,437,970 ReichMar. 16, 1948 Certificate of Correction Patent No. 2,466,230. April 5,1949. HAROLD GOLDBERG Itis hereby certified that error appears in theprinted speciication of the above numbered patent requiring correctionas follows:

Column 2, line 28, for +f(t) read it);

and that the said Letters Patent should be read with this correctiontherein that the same may conform to the record of the case in thePatent Oce. Signed and sealed this 2nd day of August, A. D. 1949.

THOMAS F. MURPHY,

Am'atant Uommaonr of Patents.

